Is it his or not??

I’m writing this for general answers do not need any negative comments as this is a hard enough situation as it is. Thank you. Soo me and my now fiancé had been dating for almost a year and a half we broke up for a period of 5 months during which time he was dating someone else. We got back together this past year December 11 2020. The last time he was sexually active with his ex was December 10th yes a day before we got back together! So fast forward to today we are now February. 4th! She told him she is pregnant we found out on January 29th that she was 5 and a half weeks and that was a confirmation from a drs office and a few days prio she had taken a at home test. The question is..is it my fiancé’s or is there no way it’s his? This has had me stressing out so much bc since we’ve been together prior the break up we had been trying to have a baby and it’s just a really hard situation.